Abstract

Development of early detection technologies and new therapeutic reagents has significantly improved the quality of treatments in breast cancer patients. However, clinical regression of primary breast tumor by the combination of surgery, hormonal, chemo and radiation therapy does not always extend patients survival. A substantial number of patients develop local and/or distant relapse overtime. The conventional explanation of these observations is that cancer cells acquire resistance during the treatment so their sensitivity to the therapy is reduced. Our previous studies have shown that breast cancer cells in process of invasion deactivate their apoptosis and cell cycle signaling pathways, suggesting they might be intrinsically resistant to radiation and cytotoxic reagents. In this study, we have successfully isolated the cancer cells from four different steps of the metastatic process: the average primary tumor cells, invasive tumor cells (isolated by our in vivo invasion assay), circulating tumor cells collected from blood and metastatic cells separated from lung metastasis. These cells were treated with Tamoxifen, Tamoxifen /Doxorubucin combination and ionizing radiation. Then the percentage of dead and apoptotic cells was quantified to evaluate the response to these different treatments. Interestingly, although these breast cancer cells were isolated from MMTV-PyMT mice that have had no contact with any drug or radiation, the invasive tumor cells and circulating tumor cells were resistant to all three types of treatments, while the primary tumor cells and lung metastatic tumor cells are equally sensitive. To investigate the mechanism behind this observation, we measured the expression of genes in DNA repair pathways and discovered that genes involved in repairing both single and double strand DNA break were transiently upregulated in invasive tumor cells and circulating tumor cells. The DNA repair genes were shown to predict the metastasis development in breast cancer patients when compared to a public microarray data base containing both gene expression and patient metastasis free survival data. We speculate that an elevated DNA repair activity and shutdown of cell cycle and apoptosis together contribute to drug and radiation resistance in the invasive tumor cells. To address this speculation, we chose a small molecule inhibitor of protein phosphatase IIA named LB1.2, which targets the DNA damage/repair pathways and accelerates the cell cycle, and combined it with conventional drug and radiation treatments. LB1.2 completely reversed the resistance of invasive and circulating tumor cells to both drug and radiation treatment mentioned above. In conclusion, we discovered that breast tumor cells in the act of invasion and in circulation are resistant to radiation and drug treatment because of a blocked cell cycle progression.